Analysis of spatial filtering in phase-based microwave measurements of turbine blade tips

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Holst, Thomas Arthur
Kurfess, Thomas R.
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In-process turbine monitoring has been a subject of research since the advent of gas turbines; however, it is difficult because it requires precision measurements to be made at high speeds and temperatures. The measurement of turbine blade tips is especially intriguing because of the potential it holds to greatly increase the efficiency of engine operation and maintenance. Tip-to-casing clearance is one of the major sources of inefficiency in a turbine and monitoring of this clearance would allow active tip-clearance control systems to be implemented. Also, analysis of engine wear through vibration monitoring may increase the effectiveness of engine maintenance and repair. A sensor recently developed at Georgia Tech could answer this challenge. The sensor operates by measuring the phase change of reflected microwaves to measure blade tip displacement. It is robust even in the harsh turbine environment. However, in sensor measurements, the microwave beam pattern causes a phenomenon called spatial filtering to occur, which may compromise the precision of measurements. Since the beam is not a thin line reflecting off a single point on the turbine blade, measurements are a weighted average of measurements along the entire surface within the field-of-view of the sensor. The net effect is a blurred measurement. In measuring turbine blades, only the tip is vital, so the blurring in between blades is not extremely detrimental. However, changing measurement geometry affects the amount of spatial filtering and hence the accuracy of the measurement. This thesis presents a detailed analysis of this phenomenon and especially its effect on turbine blade tip clearance measurements. A design of experiments is presented to qualitatively understand the effect of geometric factors on tip measurements. Along with experimentation, a robust, three-dimensional, ray-tracing, electromagnetic model is presented which was developed to further understand spatial filtering and to analyze specific geometric factors in the measurement of turbine blades. The research shows that microwave measurements may still be made to sufficient accuracy even considering the effect of spatial filtering, and by quantifying spatial filtering in measurements, it may be possible in to glean additional useful data from measurements.
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